This invention relates to cutting elements for use in a rock bit and more specifically to cutting elements which have a cutting table made up of segments of an ultra hard material.
A cutting element, such as a shear cutter shown in FIG. 1, typically has a cylindrical tungsten carbide substrate body 10 which has a cutting face 12. An ultra hard material cutting table 14 (i.e., layer) is bonded onto the substrate by a sintering process. The ultra hard material layer is typically a polycrystalline diamond or polycrystalline cubic boron nitride layer. During drilling, cracks form on the polycrystalline ultra hard material layer. These cracks are typically perpendicular to the earth formation being drilled. These cracks grow across the entire ultra hard material layer causing the failure of the layer and thus of the cutter. Growth of these cracks result in chipping, laminar type spalling and exfoliation. As such, there is a need for a cutting element having a cutting table that is capable of resisting crack growth.
The present invention is directed to a cutting element having a cutting table which is formed from segments of an ultra hard material. Preferably, some of the segments are made from finer grade of ultra hard material while the remaining segments are made from a coarser grade of ultra hard material. The segments alternate from a finer grade to a coarser grade across the cutting face of the cutting element. It is preferred that the finer grade material makes contact with the earth formation. As such, preferably, a finer grade segment makes up the edge of the cutting table making contact with the earth formation.
In an alternate embodiment, some of the segments are made from a first type of ultra hard material such a diamond, while the remainder of the segments are made from a second type of ultra hard material such as cubic boron nitride. With this embodiment, the segments form the cutting table and alternate from the first type of ultra hard material to the second type across the cutting table.
It is preferred that the segments are high shear compaction sheet segments which are formed by slitting a high shear compaction sheet. The segments forming the cutting table can be linear and parallel to each other. they may be concentric ring-shaped strips or spiraling strips. Moreover, two sets of strips may be employed to form the cutting table wherein the strips within each set are parallel to each other and wherein the first set is angled relative to the second set of strips.